This invention generally relates to golf balls, and, in particular, is directed to an improved low spin golf ball having a soft core and a soft cover.
Today, golf balls are generally available as solid and wound balls. Solid golf balls, used by the typical amateur golfer, provide maximum durability and distance. These balls have a core formed of a solid sphere of one or more layers. Typically, these balls have hard cores for high initial velocity and hard covers for low spin and durability.
A number of polymers, such as polybutadiene, natural rubber, styrene butadiene, and isoprene, are used in fabricating the solid cores. Today, golf ball solid cores are predominantly made of polybutadiene. Moreover, in order to obtain the desired physical properties for golf balls, manufacturers have added cross-linking agents, such as metallic salts of an unsaturated carboxylic acid. The amount of cross-linking agent added is typically about 20 to 50 parts per hundred parts of polybutadiene. Most commonly, zinc diacrylate (ZDA) or zinc dimethacrylate are used for this purpose. Of these two cross-linkers, zinc diacrylate has been found to produce golf balls with greater initial velocity than zinc dimethacrylate.
Typically, about 5 to 50 pph (parts per hundred) of zinc oxide (ZnO) is also added to the composition. This material serves as both a filler and an activation agent for the zinc diacrylate/peroxide cure system. The zinc diacrylate/peroxide cure system, which is well known to those of ordinary skill in this art, cross-links the polybutadiene during the core molding process. The high specific gravity of zinc oxide (5.57) can serve the dual purposes of adjusting the weight of the golf ball, in addition to acting as an activation agent.
As zinc oxide is known to be an environmentally unfriendly material, it would be advantageous to eliminate or at least substantially reduce the amount of this material from the manufacturing process. However, when the zinc oxide is eliminated from the composition described above, there is a reduction in cure enhancement, which results in less cross-linking and a corresponding reduction in compression and velocity. This result provides a ball with a softer feel, but the resulting ball has substantially less than the maximum velocity allowed by the USGA standard.
Therefore, it would be advantageous to provide a golf ball core composition with an activation agent other than zinc oxide, i.e., wherein all or at least some of the zinc oxide commonly present was eliminated, which would provide a ball with the lower compression, as noted above, but would maintain the velocity and distance of a high compression ball.
Wound balls, which are generally preferred by better players have higher spin characteristics and softer feel. Wound balls include either a solid rubber or a liquid filled center that is covered by many meters of elastic windings. Such cores are thereafter encased in a cover formed of SURLYN(copyright), polyurethane, or balata rubber. The winding and softer covers provide three-piece balls with higher spin rates and more control for better golfers.
Regardless of the form of the ball, players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers generally strive to produce golf balls with initial velocities that approximate the USGA maximum of 77.7 m/s or 255 ft/s as closely as possible.
Golf ball manufacturers are also concerned with varying the level of the compression of the ball, which is a measurement of the deformation of a golf ball or core in inches under a fixed load. Higher velocity on impact, and, hence, greater distance, is generally achieved by increasing the golf ball compression. Higher golf ball compression also generates a harder xe2x80x9cfeelxe2x80x9d. Moreover, harder balls must also have a hard cover to keep the driver spin rate down.
As stated above, better players generally prefer the wound golf balls that have soft covers. The soft cover provides increase spin and feel around the greens. However, these players are traditionally giving up distance off the tee because they generate too much spin.
Golf ball manufacturers are continually searching for new ways in which to provide golf balls that deliver the maximum performance for golfers of all skill levels. They seek to discover compositions that provide the distance performance of hard balls and the feel of approach shot spin of softer balls.
The invention relates to a solid golf ball having a diameter of less than about 1.70 inches, including a core formed from a rubber base material having a resilience index at least about 40 and an outer diameter of at least about 1.58 inches, and a cover disposed about the core formed of a material having a Shore D hardness of less than about 80, preferably less than about 70, more preferably less than about 65.
In a preferred embodiment, the core has an outer diameter at least about 1.59 inches. In another embodiment, the rubber base material has a Mooney viscosity of greater than about 45. Preferably, the rubber base material has a Mooney viscosity of about 50 to about 70.
In another embodiment, the core has a COR of at least about 0.75, preferably at least about 0.80, more preferably at least about 0.81. In yet another embodiment, the core has a compression of less than about 90, preferably less than about 80. Preferably, the core has a compression of less than about 75, more preferably about 60 to about 75.
In another embodiment, the core comprises a filler material having a specific gravity of greater than about 5. Preferably, the core comprises a filler material having a specific gravity of greater than about 10. In another embodiment, the filler material includes tungsten.
In still another embodiment, the cover is formed of a material having a hardness of up to about 65 Shore D, preferably up to about 60 Shore D, more preferably up to about 55 Shore D. Preferably, the cover is formed of a material having a hardness of up to about 50 Shore D. In an additional embodiment, the cover includes a blend of at least one ionomer and at least one metallocene-catalyzed polymer. Preferably, the cover comprises at least about 10 weight percent metallocene-catalyzed polymer, and more preferably the cover comprises about 15 to 60 weight percent metallocene-catalyzed polymer.
In an additional embodiment, at least one of the core or cover is formed of at least about 80% trans-isomer polybutadiene. In yet another embodiment, at least one of the core or cover is formed of about 10% to 80% trans-isomer polybutadiene. In still another embodiment, the core comprises a rubber material having a molecular weight of greater than about 200,000. Preferably, the core comprises a rubber material having a molecular weight of greater than about 300,000. In another embodiment, the core includes a material with a Mooney viscosity of at least about 40 and a filler including regrind.
The invention also relates to a two-piece golf ball having a diameter of less than about 1.70 inches, including a core formed from a rubber base material having a molecular weight of at least about 300,000 and an outer diameter of at least about 1.58 inches, and a cover disposed about the core formed of a material having a Shore D hardness of less than about 65.
In one embodiment, the rubber base material has a Mooney viscosity of greater than about 40. In another embodiment, the core has a core of at least about 0.80. In still another embodiment, the core has a compression of less than about 75. In a further embodiment, the core includes a filler material having a specific gravity of greater than about 10. In yet another embodiment, the cover is formed of a material having a hardness of up to about 55 Shore D.
The invention further relates to a golf ball having a diameter of less than 1.70 inches, including a core having a compression of less than about 75 and an outer diameter of at least about 1.58 inches, and a cover disposed about the core formed from a material having a Shore D hardness of less than about 60.
In one embodiment, the core is formed from a rubber base material having a Mooney viscosity of greater than about 40. In another embodiment, the core has a COR of at least about 0.80. In still another embodiment, the core includes a filler material having a specific gravity of greater than about 10.